Physiologically active substances, in particular, glycoproteins or antibodies have been recently approved as various kinds of biopharmaceuticals, and more candidate substances are currently under development (Non-Patent Literature 1). For this reason, it is expected that production of physiologically active substances such as glycoproteins, antibodies or the like using cells will be more actively performed.
With respect to preparation of an aqueous solution for cell culture which is essential for the production of these physiologically active substances using cells, sterilization of a culture medium is required to ensure product and process safety. Until now, a 0.2 μm membrane filtration process has been widely used for removing microorganisms in the preparation of the aqueous solution for cell culture. In recent years, a 0.1 μm membrane filtration process has been proposed for mycoplasma removal (Non-Patent Literature 2). Therefore, excellent membrane filterability of the aqueous solution for cell culture is increasingly required and it is a problem.
Compared to small-molecule drugs, biopharmaceuticals require high production costs, which is also a problem faced by pharmaceutical industries (Non-Patent Literature 3). So far, efforts to reduce the cost has been made by achieving productivity improvement resulting from enhancement of some components and addition or enrichment of new components of the aqueous solution for cell culture, but on the other hand, the difficulty of achieving the excellent membrane filterability of the aqueous solution is increasing. Because of restrictions on facilities and scale-up of production equipments to meet market needs, there is also a need for highly versatile aqueous solutions for cell culture which can be more stably membrane-filtered in a short time.
Alteration of filtration equipments, addition of filtration membrane or replacement of filtration membrane material, or improvement of dissolution conditions such as temperature during preparation of the aqueous solution for cell culture have been tried to improve the amount of membrane filtrate or membrane filterability of the aqueous solution (Patent Literature 1, Non-Patent Literatures 4 and 5). However, the improvement of membrane filterability of the aqueous solution for cell culture by increasing the membrane area is not industrially preferred in terms of restrictions on costs and facilities. Further, the improvement of dissolution conditions did not bring about remarkable improvement of membrane filterability of the aqueous solution for cell culture.
Chelating agents such as citric acid, malic acid, ethylenediaminetetraacetic acid or the like have been widely known as one of the components contained in the aqueous solution for cell culture (Patent Literatures 2 and 3). Sialic acid is also known as a substance having a chelating function (Non-Patent Literature 6). However, there have been no reports of a method for improving membrane filterability of the aqueous solution for cell culture by use of chelating agents.